1. Field of the Invention
This invention relates to a nitride semiconductor crystal producing method, a nitride semiconductor epitaxial wafer, and a nitride semiconductor freestanding substrate.
2. Description of the Related Art
For GaN-based material growth, a vapor phase growth method, such as metal organic vapor phase epitaxy (MOVPE) or hydride vapor phase epitaxy (HVPE), is mainly used. In these growths, a seed crystal substrate, such as a sapphire substrate, SiC substrate, or nitride semiconductor substrate, is installed on a holder, a tray or the like, and a raw material gas is fed to the seed crystal substrate, to grow a nitride semiconductor. Specifically, in the metal organic vapor phase epitaxy, the nitride semiconductor is grown over the seed crystal substrate, by feeding an organometallic gas, such as trimethyl gallium (TMG), trimethyl aluminum (TMA) or the like, and ammonia. Also, in the hydride vapor phase epitaxy, the nitride semiconductor is grown over the seed crystal substrate, by feeding a group III raw material gas, such as gallium chloride (GaCl) gas, aluminum chloride (AlCl, AlCl3) or the like, and ammonia to the seed crystal substrate installed on the holder, the tray or the like.
As the seed crystal substrate, a heterogeneous substrate made of sapphire, SiC, Si or the like, or a nitride semiconductor freestanding substrate made of GaN, AlN or the like is used. Over these seed crystal substrates, crystal growth is performed so that typically a group III polar c-face such as a Ga face is a front side, and that surface is used for device formation.
A problem arises in the nitride semiconductor crystal growth: When the grown layer of the nitride semiconductor becomes thick, cracking tends to occur in the grown layer.
In particular, in the thin film growth over the heterogeneous substrate, this problem arises when the thermal expansion coefficients of the seed crystal substrate and the nitride semiconductor layer grown thereover are significantly different. For example, in the growth of the GaN layer over the sapphire substrate, cracking tends to occur when the thickness of the GaN layer exceeds 5 to 6 μm. In the growth over the SiC substrate or over a Si substrate, even the thinner GaN layer of the order of 2 to 3 μm is likely to crack unless a complicated stress relaxation layer or the like is built therein. Such a limit to the growable GaN layer thickness causes such various inconveniences that types of applicable devices are limited, or enhancement of device properties is prevented.
The cracking mechanism in the thin film growth over the heterogeneous substrate is typically described as follows: Even when the GaN layer is grown over the heterogeneous substrate at a growth temperature (up to 1000 degrees Celsius), no significant stress occurs during the growth. After the growth, however, returning the temperature of the nitride semiconductor epitaxial wafer formed with the GaN layer over the heterogeneous substrate to room temperature causes stress due to a difference between thermal expansion coefficients, leading to bimetallic warping of the nitride semiconductor epitaxial wafer. In the GaN layer over the sapphire substrate, at thicknesses exceeding 5 to 6 μm, the stress exceeds the critical value, causing the GaN layer to crack. Herein, the critical thickness value (critical film thickness) of the GaN layer over the sapphire substrate is assumed to be 5 to 6 μm, but in practice that critical value varies according to growing apparatus used, growth conditions, etc., and as it stands, it is difficult to determine the definite growth thickness at which cracking occurs.
As well as cracking during thin film growth on the order of a few μm thickness described above, the occurrence of cracking has been a major problem even in the growth of the nitride semiconductor freestanding substrate having a few 100 μm to a few mm thickness.
There are two ways to fabricate the nitride semiconductor freestanding substrate: One way is to use the heterogeneous substrate as the seed crystal as in the case of the above-described thin film, and another way is to use the nitride semiconductor freestanding substrate as the seed crystal.
In the freestanding substrate production onto the heterogeneous substrate, the use of for example, a Void-Assisted Separation (VAS) method disclosed by Patent document 3 allows a void layer to mitigate the stress between the heterogeneous substrate and the nitride semiconductor layer grown thereover. For this reason, the occurrence of cracking due to the stress between the heterogeneous substrate and the nitride semiconductor layer as in the above described case of thin film growth is suppressed, and the nitride semiconductor layer having a film thickness of the order of 100 μm can be grown without cracking. However, even in this case, the situation at the initiation of growth is the same as in the case of thin film growth: The growth on a face tilted from the c-face occurs around an outer end from the beginning of the growth. This leads to the occurrence of stress. At thicknesses of the nitride semiconductor layer exceeding 100 μm, cracking tends to occur during the growing and cooling. Since the general thickness of the semiconductor wafer is required to be on the order of 400 μm to 1 mm from the point of view of ease of handling, it is necessary to grow such a very thick nitride semiconductor layer in the growth of the nitride semiconductor freestanding substrate as well. For this reason, the occurrence of cracking due to the presence of stress in the outer end is a serious problem which excessively reduces yield in the production of the nitride semiconductor freestanding substrate.
In order to remove a nitride semiconductor polycrystal adhering to an inner wall of a reactor or around a susceptor to place an underlying substrate, Patent document 1 (JP-A-2007-320811) has disclosed a method by introducing an etching gas into the reactor after growth, so that nitride semiconductor substrate cracking during cooling and damage to the inner wall of the reactor are suppressed. When growing the thick nitride semiconductor layer, the method of Patent document 1 has failed to reduce the occurrence of cracking during the growth, and has failed to increase the yield of the nitride semiconductor crystal, and has made small the obtainable area of the nitride semiconductor substrate due to an excess of the undesirable nitride semiconductor polycrystal around the susceptor. Also, Patent document 2 (U.S. Pat. No. 6,632,725) has disclosed a method by feeding HCl (hydrogen chloride gas) into a reactor as an etching gas during GaN growth, to reduce GaN adhering to an inner wall of the reactor. Further, in order to reduce warpage of a laminate with a GaN layer grown over a sapphire substrate, Patent document 4 (JP-A-2007-106667) has disclosed a method by nitriding and hydrogen chloride gas etching of the sapphire substrate, to form an uneven structure of an aluminum nitride in the surface of the sapphire substrate, so that GaN is grown over the sapphire substrate having that uneven structure of the aluminum nitride.    Patent Document 1: JP-A-2007-320811    Patent Document 2: U.S. Pat. No. 6,632,725    Patent Document 3: JP-A-2004-039810    Patent Document 4: JP-A-2007-106667